This invention is related to the composition of an elastomeric copolymer, and, additionally, to the use of such an elastomeric copolymer to prepare dip-formed articles.
Thin-walled, extensible articles such as gloves, condoms, and other products have long been made from natural rubber. In normal productions, such articles are formed from natural rubber latex, a naturally occurring emulsion of rubber and water, with added stabilizing agents and vulcanizing chemicals. A form of the appropriate shape, previously coated with a coagulating solution in some cases, is dipped into the latex mixture once or several times to build up a layer of the desired thickness. The water is allowed to evaporate, leaving a solid rubber film. The film must be vulcanized to provide adequate mechanical and physical properties.
Natural rubber has many advantages in these applications, being strong and highly elastic and having good xe2x80x9ctactilityxe2x80x9d or feeling to the user. Natural rubber has several shortcomings, such as susceptibility to xe2x80x9cpinholesxe2x80x9d therethrough, rapid attack by ozone which causes scission cracking, and oxidative attack during storage which causes cracking and destroys the physical integrity of the product. Natural rubber is also not hypoallergenic due to the residual surfactants, vulcanizing agents, stabilizing agents, antioxidants, and/or protein materials in the rubber. Persons who are particularly susceptible to irritation or sensitization, or use the rubber products for extended periods of time may experience allergic reactions.
Various types of synthetic elastomeric polymer products have been developed for use in thin articles produced by dip forming. Synthetic rubber compositions can be dissolved in solvents to form a true solution, so that pinholes are much less likely to be present. Many available synthetic rubber compositions have various other shortcomings, including unacceptable tactility. While each may meet some of the requirements, most do not have the required combination of strength, tactility, resistance to environmental damage, and hypoallergenicity required for many products such as examination and surgical gloves, condoms, and other medical products that are to come into contact with the human body.
An important advance in the art of synthetic elastomeric polymer products is described in U.S. Pat. Nos. 5,112,900 and 5,407,715. These patents disclose the preparation of specific styrene-ethylene/butylene-styrene (S-EB-S) block copolymer solutions and their use in the dip forming of articles. The resulting articles have excellent elastomeric properties for use in gloves, condoms, and other products. They exhibit low incidence of pinholes, good resistance to environmental damage such as oxidation and ozonation, and hypoallergenicity.
There is, however, always a need to further improve the manufacturability of articles made of such formulations and the process economics. The present invention fulfills this need, and further provides related advantages.
The present invention provides an elastomer liquid solution, process for preparation of elastomeric articles, and articles. The approach of the invention overcomes manufacturing complexities and uniformity problems associated with the technique of U.S. Pat. Nos. 5,112,900 and 5,407,715, while retaining their advantages.
In accordance with the invention, an elastomer liquid solution consists essentially of a block copolymer component comprising a single S-EB-S block copolymer, wherein the S-EB-S block copolymer has at least about 15 weight percent of styrene and blocks, and wherein the molecular weight (weight average molecular weight is used throughout) of the styrene end blocks is at least about 7,000 Daltons and the molecular weight of ethylene-butylene midblocks is at least about 60,000 Daltons. The solution further includes a plasticizer in an amount sufficient to provide tactility in dip formed products made from the composition, and a solvent in an amount sufficient to form a stable solution of the block copolymer component and the plasticizer and to permit dip forming of products from the liquid solution.
The amount of plasticizer to be used is selected in conjunction with the nature of the S-EB-S copolymer. In particular, the volume percentage of the styrene end blocks based upon the total of the S-EB-S block copolymer and plasticizer (termed herein the xe2x80x9cend block phase volume percentagexe2x80x9d, but which can also be termed a xe2x80x9chard domain phase volume percentagexe2x80x9d). Preferably, the amount of plasticizer is such that the end block phase volume percentage of the styrene end blocks is less than about 20 percent. Thus, the higher the percentage of styrene end blocks in the S-EB-S block copolymer, the larger the amount of plasticizer that may be used. On the other hand, the amount of plasticizer may not be so large that the strength of the final product is reduced below acceptable levels as required for each product.
In accordance with a processing aspect of the invention, articles are made from this liquid solution by dip forming. In this approach, there is furnished a liquid solution as described above, but wherein the molecular weight of the S-EB-S block copolymer is less than about 175,000 Daltons. A form is dipped into the solution and withdrawn, and the solvent is evaporated from the film on the form, leaving a coherent extensible film on the form in the shape of the form.
The present approach is operable to produce articles such as gloves and condoms of high quality. The articles have a combination of good strength and elasticity, together with a tactility comparable to natural as shown by low deformation stress at 50-500% elongation and highly elastic recovery. Thin goods formed of the elastomers avoid the problem of pinholes, or, alternatively stated, have a high degree of impermeability. The elastomeric composition is not prone to scission cracking upon exposure to ozone or cracking upon aging, and is well suited for use in thin goods formed by dipping, such as gloves and condoms.
These same qualities are obtained in articles made according to U.S. Pat. Nos. 5,112,900 and 5,407,715, whose disclosures are incorporated by reference. However, the approach of these patents requires the use of at least two, and preferably three, different S-EB-S copolymers. The use of different copolymers adds complexity to the manufacturing operation, and requires care in mixing and preparation of solutions. Relatively minor variations in either the properties of any of the as-purchased S-EB-S copolymer materials or their mixture proportions in solution result in a change in the viscosity and other properties of the solution. The operating parameters of the dip-forming line (e.g. solution temperature, form withdrawal rate, etc.) must be adjusted responsively, and, even then, there may result a higher incidence of defects, such as holes in the product and interfinger webs in gloves, in the finished products both as the operating parameters are responsively changed and equilibrium is again achieved, and during extended periods of operation. Also, of course, the use of multiple copolymer components requires greater logistical care in ordering, handling, and storing of the components. These difficulties are not insurmountable, and the assignee of the ""900 and ""715 patents, and of the present invention, has built a successful business using the approach of these patents. However, the present approach offers the important advantage of reduced manufacturing complexity and improved product yields by requiring only a single S-EB-S block copolymer.
Additionally, when two or more copolymers are mixed together in solution, there is a corresponding number of different molecular weights in the final product. Each of the individual copolymers is usually manufactured with a relatively narrow distribution in molecular weight, so that, when two copolymers are mixed together in solution, there are two different primary modes in the molecular weight distribution. The result is a degree of nonuniformity in the final polymer network, although this nonuniformity has been acceptable in products made according to the ""900 and ""715 patents. In the present approach using only a single copolymer, on the other hand, the microstructural uniformity of the final product is improved.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.